1. Field of the Invention
The present invention relates to a hand drill including a housing, a chuck provided at a front, in a drilling direction, end of the housing for receiving a drill or chisel tool, a rotary drive arranged inside the housing for driving the chuck, together with the drill or chisel tool, a compressed air-operated hammer mechanism for generating axial blows to be applied to the drill or chisel tool and having a pneumatic cylinder with at least one inlet opening and at least one discharge opening, a die member for imparting the axial blows, which are generated by the hammer mechanism, to the drill or chisel tool and extending through a front limiting surface of the pneumatic cylinder, and a percussion piston displaceable in the pneumatic cylinder upon being impinged by compressed air for intermittently applying axial blows to the die member, and a reversing valve for connecting the hammer mechanism with a source of compressed air.
2. Description of the Prior Art
In addition to hand-held drills provided with electro-pneumatic hammer mechanisms or mechanical hammer mechanisms such as ratchet hammer mechanisms, spring-actuated hammer mechanisms and cushioned cam hammer mechanisms, also are used hand-held drills having a compressed air-operated or servo-pneumatic hammer mechanisms which include a pneumatic cylinder in which a percussion piston is arranged. The percussion piston is displaceable by the compressed air and periodically applies axial blows to a die member which transmits the blow to a tool secured in the chuck of the hand-held drill. In the known compressed air-operated hammer mechanisms, a reversing valve is provided between the pneumatic cylinder and the source of the compressed air, e.g., a compressor located in the drill housing. The reversing valve provides for alternating supply of the compressed air to the pneumatic cylinder and the discharge of the compressed air from the pneumatic cylinder for reciprocating the percussion piston in the pneumatic cylinder chamber. The operation of the reversing valve is controlled by end switches which are actuated in front and rear end positions of the percussion piston. The switching of the reversing valve proper is then effected by appropriate mechanical, electrical means or by communicating to the reversing valve the compressed air through control conduits.
The drawback of the known compressed air-operated hammer mechanisms consists in that they have a large dead volume which must be reloaded between each pressurized condition of the pneumatic cylinder and each unpressurized condition of the pneumatic cylinder. This adversely affects timely deceleration of the percussion piston and, thereby, a predetermined blow frequency. Further, the permanent reloading of the large dead volume leads to large energy losses. The known compressed-air operated hammer mechanisms have at least one reversing valve and several end switches. Such an arrangement causes a time delay in switching from one condition of the reversing valve to another condition thereof, which adversely affects the blow power. Further, the energy of a single blow and the frequency of the generated axial blows can only be controlled by the pressure acting on the hammer mechanism to a very small extent.
Accordingly, an object of the present invention is to eliminate the drawbacks of conventional compressed air-operated hammer mechanisms and to provide a hammer mechanism in which the time delay in switching of the pneumatic cylinder between its pressurized and unpressurized conditions is eliminated to a most possible extent.
Another object of the present invention is to provide a hammer mechanism in which the energy necessary for reloading of the dead volume is reduced, and the energy balance for generating axial blows is substantially improved.
A further object of the present invention is to provide a hammer mechanism which would provide greater possibilities for adjusting the energy of single blows and the blow frequency.